1. Field of the Disclosure
The present disclosure generally relates to apparatus and methods to test drilling fluids. More particularly, the present disclosure relates to methods and apparatus to measure the loss of drilling fluids through selected formations over time. More particularly still, the present disclosure relates to apparatus and methods to test the effectiveness of loss prevention materials entrained in drilling fluids against selected formations.
2. Background Art
When drilling in depleted zones or otherwise weak formations, wellbore instability and formation damage are major concerns for operators. Because of the economic incentives to extract as much of the hydrocarbon deposits from subterranean reservoirs as possible, it is often necessary to drill in depleted zones and through weakened formations. However, drilling through such highly depleted zones may increase the risk of losing drilling fluid (i.e., drilling “mud”) through fractures or other features of the formation. Therefore, it is advantageous to drill using a drilling fluid optimized for a particular formation such that the loss of drilling fluid is minimized.
As such, drilling fluids designed for depleted zones should be capable of maintaining or enhancing wellbore stability while simultaneously preventing severe drilling fluid matrix (i.e., the base fluid) losses by healing natural or induced fractures. In addition to preventing severe matrix losses, loss control may also reduce the risk of differential sticking. The rheology of the base drilling fluid plays a role in flow through porous media, particularly in the initial stages of contact between the fluid and the freshly drilled formation.
Fluid loss during this initial stage is referred to as “spurt loss” and may continue until an effective filtercake or bridge is built-up to act as a barrier to further losses. In high permeability formations, or where induced or natural fractures exist, whole drilling fluid may be lost into the formation during this stage which, if left uncontrolled, may continue and lead to severe mud losses and loss of the ability to control the well.
However, most of these problems may be averted (or their effect minimized) by the inclusion of an effective loss prevention material (“LPM”) in the drilling fluid mixture. However, because formations and wellbores are unique, no single LPM configuration (e.g., material, size, concentration, etc.) will be optimal for all drilling conditions. Therefore, it would be highly desirable to develop methods and apparatus to experimentally test varying drilling fluid and LPM compositions on a variety of formation structures.
Formerly, a permeability plugging test (“PPT”) was performed to measure matrix fluid loss characteristics. In a PPT, matrix loss characteristics were evaluated on relatively thin ceramic discs having large pore throat sizes. As such, the flow path through the ceramic media was relatively short and generally did not allow the mud rheology sufficient time to exert control over the loss of the fluid matrix.
Therefore, a better alternative to the PPT would be highly desired. Such an alternative would involve a matrix loss test over a relatively longer test medium such that the ability of a drilling fluid's LPM to control loss beyond the “spurt loss” stage could be measured. Such an alternative would provide a flow path of adequate length so that the fluid rheology may build matrix loss resistance through gelling or the filtercake formation.